Ultra high frequency voltmeter



May 26, 1942. Y 0. E. DOW 2,284,379

ULTRA HIGH FREQUENCY voLTMETER v Filed Dec.`29, 1959 l INVENTOR. URV/LE E. DOW

A TTORN EY.

Patented May 269 E94@ Orville E. ma', Rocky Point, N.' Y., assigner to Radio Corporation oi America, a corporation oli Delaware ,encuentren menuba- 29, i939, serial No. 311,507

(ci, ivi-e) Bd maints.

This invention relates to improvements in voltmeters and voltage standards for ultra high irequencies and, more particularly, to such instruments for frequencies greater than 300 megacycles.

The present practice is to use thermionic diodes or thermo-couples in conjunction with the direct current meters and multiplying resistors to measure frequency voltages. The thermionic diode in conjunction ywith the direct current instrument is a very valuable instrument for most alternating current measurements but is subject to errors at frequencies above 300 megacycles due to the transit time of electrons within the diode and also errors .due to the inductance of the electrode leads. Furthermore, lthe thermionic diode also requires a power supply for the ila.-

ments. Thermo-'couple meters, in spite of the recent improvements, are still subject to considerable error at ultra high frequencies unless great care is exercised in avoiding effects of stray capacities and elds.

An object, therefore, of the present invention is to provide an instrument which is free from the above mentioned sources of error.

Another. object of the present invention is to provide an ultra high frequency voltmeter which is not eiected by stray capacities across the multiplying impedances which must be used.

Still another object of the present invention is to provide an ultra. high frequency voltmeter which is conveniently-med, which may be readily calibrated and which disturbs the circuit to which it is connected to a negligible extent.

The foregoing objects are attained by the provision, in accordance with the present invention, of a thermo-couple witha novel arrangement of uncoupling the heater leads from the thermocouple leads so that the latterare completely shielded from radio frequency nelds. A novel form of multiplier is also provided between the source and the-thermo-couple heater element. Furthermore, the present invention includes 'an arrangement'whereby the eifect of stray capacities across the multiplying impedance is reduced.

Further objects and features o5.' the present inventlon will become apparent from the following detailed description, which is accompanied by a drawing in which Figure 1 shows one modication of the invention, Figure 2 shows a modincation which is useable with lower voltage ranges and Figure 3 shows still a further modlcation of the invention. I

In Figure l I have shown a section of a concentric cable transmission line composed of an outer shell i andan inner conductor 2. The length of this section of line is adjustable, by means of the telescopic joints in the shell and inner conductor, to be equal to a quarter of the length of the wave which it is desired to measure. A characteristic of a quarter wave section of transmission line. closed at one end,.is that the voltage applied at the open end of the line is transformed to current at the closed end of the line, that is, it acts as an impedance inverting network. The value of the current at the closed end of the line is equal to the voltagel at the open end divided by the characteristic impedance of the transmission line. It should be noted that shell l and conductor 2 are eectively in series across the applied voltage. Since their total length is a half wave the device does not load the circuit across which it is connected. At the y inner end of the transmission line the inner conductor 2 is connected to the outer conductor l by means of a very thin conducting cylinder 3 which preferably has the same diameter as inner conductor 2. This thin cylinder 4forms the' heater for the thermo-couple unit whichvwill later be described in more detail. The heater 3 must be securely connected to inner conductor 2 and the i end plate of outer shell i so that a full contact is made. The wallsoi the heater element 3 must be thin enough that its direct current resistance is equal to the resistance at the frequency for which the meter' is designed. The cylinder must be short enough to have a uniform distribution of current over its length. Furthermore, the

wall thickness must be uniform heating. f

Platinum is suitable lmaterial of which to make the heater element 3. To the interior of the heater element 3 is connected the thermo-couple junction 5 of conventional construction. A directl current meter 9 is connected to the thermo-couple junction 5 .by suitable leads. -.I t will be noted by an inspection of Figure lthat the thermo-couple junction 5 and the leads connected thereto are located inside the tube 3 `so that they are completely shielded from the radio frequency'neld which is necessarily conned to the space between the inner conductor 2 and the outer shell i. The inner conductor 2 is mechanically supported by bridge piece fl which may be made of either insulating or conducting material, as desired. The parts l and 2 .of the quarter wave transformation line each are shown as being made of telescopic sections so that the lengths thereof may be adjusted to an odd numto prevent spotty ber of quarter wavelengths over a wide range of frequencies. Y

In operation, the voltmeter'as shown in Figure 1 may be calibrated by noting the reading of the direct current. meter 3 when a known low frequency or direct current ows through the transf formation line from the source applied at the open end of the line. When an unknown voltage of a wavelength equal to four times the length of the inner conductor 2 is applied at the open end of the transformation voltage nodal point is determined from the reading of previously calibrated meter 3. 'Ihe unknown voltage at the open end of the line is found by multiplying this current by the characteristic impedance of the transmission line. If the'unknown voltage is designated by the quantity En, the impedance of the'transformation line by the quantity Z and the current atthe closed end of the the voltage Eo is then equal line the resultant current at thel age standard the value of current required to make the wire glow to a predetermined amount may be determined at low frequencies or with direct current. 'Ihe ultra high frequency generator is then connected to the open' end of the line and the voltage raised until the wire again shows the same color. The voltage at the inputof the line is then equal to the current in wire 33 multiplied by the characteristic impedance of the line. The standard voltage may be varied by utilizing different diameters vand resistivities of wire 33 and diierent values of characteristic impedancel l of the transformation line. Furthermore, within certain limits the temperature of wire 33 may be determined by an optical pyrometer. The readings of the pyrometer may be directly calibrated in voltages in the open end of'transformation line I, 2. If desired, the temperature of wire 33 may be determined by a thermometer, which may then be calibrated in terms of voltage at the open end of line I, 2. The modification shown'in Figure 3 is not useable for such large values of current as the modification shown in Figure 1 since-the maximum possible useable diameter of the wire 33 is limited by skin 'eect of the high frequency current causing errors as the diameter is increased.

WhileI have particularly shown and described an embodiment of the present invention, it is to be ages the arrangement shown in'Figure 2 may -be used wherein the transformation line is construct'ed in' the same way as described with reference. to Figure 1. However, in this modication l the heater 23 consists of a short length of very fine wire joining the inner end of conductor l2 and the outer shell I. In this modification since it is not possible to make the inner conductor 2 self-supporting suitable insulators 1 are. interposed between shell I and conductor 2 at the'desired intervals along the length of the transmission line.

'The thermo-couple junction I is connected to the 1 heater wire 23 in any suitable fashion and the leads therefrom are brought outthroughthe end wall of the'transformation line to direct current meter 9. In this modification the junction and 1 thermo-couple leads are exposed to radio frequency fields and it is, therefore, necessary to filter the direct current leads by means of inductances 3 and condensers I6. As in the previously clearly understood that my invention is not limited thereto but that, modifications may be made .within the scope of this invention.v

I claim: ,i 1. In combination, a Aconcentric line section having an inner conductor and an outer shell, said outer shell being closed at one end and said section being adapted to have radio frequency voltage applied to the open' end thereof, the length of said line section being equal to a quarter of the wavelength of said applied voltage. aheater element connected between said innerconductor and the closed end of said outer shell and means for deterdescribed modification, the lengthof heater 23 i must be short enough to insure uniform distribution of current and the diameter must in this modification be small enough to avoid errors due to ,skin effect. The diameter of the irmer conductor 2 must be kept small in order to reduce stray capacity across the heater element-3 between the end of conductor 2 and the end wall l.

In Figure 3 I have shown a further modification of the invention which may also be utilized in mining the relative temperature of said heater element.

, 2. In combinatioma concentric line section having an inner conductor andan outer shell, said outer shell being closed at one end andsaid section being adapted to have radio frequency voltages applied to the open end thereof, said inner conductor and outer `shell being telescopic whereby the length of said line section is adjustable to a length equal to a quarter of the wavelength of the applied voltage, a heater element connected between said inner conductor and the closed end of said outer 'shell and means for determiningthe relative temperature of said heater element.

establishing a voltage standard. The operation .is

similar to the operation of the modification shown in Figure 2, with the exception that thev'current at the voltage nodal point is determined visually by the temperature of the wire instead of using a thermo-couple element. The current through the wire 33 required to raise its temperature until it just barely glows may be definitely determined. The heater wire'33 is well shielded by the outer shell I thus avoiding any errors due to air currents cooling the wire. An aperture 3 is provided in the outer shell I as a means for observing the temperature of wire 33. To'operate this device as avoit- 3. In combination, a concentric line section having an inner conductor and an outer shell, said outer shell being closed at one end and said section'being adapted to have radio frequency voltages applied to. the open end thereof, the length of said line being equal toa quarter of the wavelength of the applied voltage, a heater element connected between said inner conductor and the closed end 'of said outer shell, a thermocouple in contact with said heater element and means connected to said thermo-couple for meas` uring the current flowing through said heater element.

4. In combination, a concentric line section having an inner conductor and an outer shell. said outer shellfbeing lclosed at one end and said section being adapted to have radio frequency voltages applied to the open end thereof, the

'length of said line being equal to a quarter of conductive tube having a diameter equal to the u the wavelength of the applied voltage, a heater element connected between said inner conductor and the closed end of said outer shell, a thermocouple in contact with said heater element, means connected to said thermo-couple for measuring the current flowing through said heater element, and means for preventing radio frequency currents from aifecting said measuring means.

5. In combination, a concentric line 'section having an inner conductor and an outer shell, said outer shell being closed at one end and said section being adapted vto have radio frequency voltages applied to the open encl thereof, the length of said line beinggequal to a quarter of the `wavelength of the applied voltage, a heater element connected between said inner conductor and the closed end of said outer shell, a thermocouple incontact with said heater, a direct current meter, connections from said direct current meter to said thermo-couple, said connections including series induct'ances and parallel capacities whereby radio frequency currents are prevented from aiecting said direct current meter.

6. In combination, a concentric line section having an inner conductor and an outer shell, said outer shell being closed at one end and said section being adapted to have radio frequency 'voltages applied to the open end i thereof, the length of said line section being equal to a quarter of the wavelength of the applied voltage, a thin conductive tube connected between said inner conductor and said closed end of the outer shell and means for determining the relative temperature of said thin conductive tube.

il. In combination, a concentric line section having an inner conductor and an outer shell, said outer shell being closed at one end and said section being adapted to have radio frequency voltages applied to the open end thereof, the length of said line section being equal to a quarter ci the wavelength of the applied voltage, a thin conductive tube connected between said inner conductor and the closed end of said outer shell, a thermo-couple within said tube and a direct current meter connected to said thermo-couple.

8. In combination, a concentric line section having an inner conductor and an outer shell, said outer shell being closed at one end and said section being adapted to have radio frequency voltages applied to the open end thereof, the length of said line section being equal to a quarter of the wavelength of the applied voltage, a thin conductive tube havinga diameter equal to the diameter of said inner conductor and connected between said inner conductor and the closed endof said outer shell, a thermo-couple within said tube and in contact with its inner surface and a direct current meter connected to said thermocouple.

9. In combination, a concentric line section having an inner conductor and an outer shell, said outer shell being closed at one end and said section being adapted to have radio frequency voltages appliedeto the open end thereof, the length of said line section being equal to a quarter of the wavelength of the applied voltage, a thin diameter of said inner conductor and connected between said inner conductor and the closed end of said outer shell, a thermo-couple within said tube and in contact with its inner surface and a direct current meter connected to said thermocouple by leads within said tube and passing through an aperturein said closed end.

l0. In combination, a transformation line sectioncomprising a pair of parallel conductors, said conductors being connected together at one end and said section being adapted to have radio frequency voltages applied to the open end thereof, the length of said line section being equal to a quarter of the wavelength of said radio frequency voltages, a heater eiement forming the connection between said pair of conductors' and means for determining` the relative temperature of said heater element.

il. in combination-a transformation line section comprising a pair or parallel conductors, said conductors being connected together at one end and said section being adapted to have radio frequency voltages applied to the open end thereof, the length oi said line section being equal to a quarter of the wavelength of said radio frequency voltages, a heater element forming the connection between said pair of conductors, a thermo-couple in contact with said heater element and means connected to said thermo-couple for measuring the current fiowing through said heater element.

i2. In combination, a transformation line section comprising a pair oi parallel conductors, said conductors being connected together at one end and "said, section being adapted to have radio irequency voltages appiied to the open end thereof, the length of said line section keinev equal to a quarter ci the wavelength of said radio frequency voltages, a heater element forming the connection between said pair of conductors, a thermo-couple in contact with said heater element, means connected to said thermo-couple for measuring 'the current fiowing through said heater element and means for preventing' radio frequency currents from affecting said measuring means.

i3. A high frequency voltmeter comprising an impedance inverting device having an input and an output, and current responsive means cony nected to said output, said device being adapted to transform a voltage applied to said input tov a current through said current responsive means having an amplitude proportionate to said voltage, said current responsive means being adapted to indicate the amplitude oi said current.

14. A high frequency voltmetercomprising an l impedance inverting device having an input and an' output, land current responsive means connected to said output. said device being adapted to transform a voltage applied to said input to a current through said current responsive means having an amplitude proportionate to said voltage, said current responsive means comprising a thermo-couple having input terminals connected to the output offsaid impedance inverting device and output terminals connected to a direct current meter.

ORVIILE E. -DOW. 

